The present invention relates to a method of securing a semiconductor chip on a base plate and a structure thereof.
Electric field electron emission cold cathodes, i.e., a field emiiter or a cold cathod have recently been developed as electron sources for electron guns or the like. FIG. 4 is a sectional view showing such an electric field electron emission cathode. Usually, the cold cathode 30 comprises an electron emission area 33 with several hundreds to several tens of thousands of projecting micro-emitters 32 formed in a circular or square array on a semiconductor substrate 31 and a gate electrode 34 formed above the micro-emitters 32 via an insulating layer 35 to apply an intense electric field to the micro-emitters 32.
In this electric field electron emission cold cathode, by applying a voltage between the gate electrode 34 and each micro-emitter 32 an electric field is concentrated at the pointed end of the micro-emitter 32 to cause emission of electrons corresponding to the potential difference between the two electrodes from the micro-emitter 32. The pointed end of the micro-emitter 32 may have a diameter of about 1 .mu.m, and in this case an application of voltage of 50 to 100 V between each micro-emitter 32 and the gate electrode 34 causes emission of about 1 .mu.A of electrons from the micro-emitter. An electric field electron emission cold cathode used for a usual cathode-ray tube has several thousands to several tens of thousands of micro-emitters. In this case, the electron emission area has a size of several hundred .mu.m because each micro-emitter has as small diameter as about 1 .mu.m.
FIG. 5 is a sectional view showing an electron gun cathode structure with electric field electron emission cold cathodes assembled therein. Each cold cathode 41 is mounted in a sleeve 43, which is secured by an insulator 44 to an outer cathode support 45. Reference numeral 40 designates bonding wires, 42 electron emission areas of the cold cathodes, and 49 connection ports. Reference numeral 46 designates a first grid, and 47 a second grid. The electron emission area 42 of each cold cathode 41 has to be in an accurate position relation to the first grid 46, because this relation determines the characteristics of the electron gun.
Japanese Laid-Open Patent Publication No. 7-161304 discloses a technique concerning a cathode structure using cold cathodes. FIG. 6(a) is a perspective view showing this cathode structure, and FIG. 6(b) is a sectional view taken along line 6-6'. In this cathode structure 50, each electric field electron cold cathode chip is secured to a cap-like chip holder 52 formed with a window on a top thereof such that its electron emission area 53 is exposed by a window 51 formed in the chip holder 52 and the chip contact area 54 is secured to a back surface edge portion of the window 51. As shown in FIG. 6(b), the chip is pushed against the chip holder 52 by an elastic connecting piece 61. A feature of this prior art structure resides in that the gate electrode of the cold cathode is wired without use of any wire bonding but by pushing the chip against the chip holder.
FIG. 7 shows a cathode-ray tube. Electron beams 78 emitted from the cold cathodes of a cathode structure 71 are converged by an electron lens of an electron gun 72 formed by a first to a sixth grids facing the cathode structure 71 to pass through a shadow mask 73 and impinge upon a phosphor screen 77, thus causing generation of predetermined colors. The phosphor dots of the phosphor screen 77 constituting pixels of various colors have a typical diameter of about 100 .mu.m. The beam spot formed by each electron beam 78 on the phosphor screen 77 should have such a diameter that it can irradiate a plurality of phosphor cells at a time.
Since a large size cathode-ray tube has a problem of spherical surface aberration at its end surface, tending to increase the electron beam spot size, it is necessary to decrease the spot diameter in size. Therefore, the electron lens assembling accuracy is extremely high, specifically several to 10 .mu.m. In other words, the assembling accuracy of the electron gun is in a range, can sufficiently allow electron beam convergence fluctuations and fluctuations of the beam spot position on the phosphor screen.
In the electron gun using cold cathodes, it is important to take into account that the focused image on the phosphor cell is a point light source image of the emission area of the cold cathode and the cold cathode mounting position accuracy in a plane perpendicular to the electron beam progress direction is directly reflected on the phosphor cell position accuracy. This means that the cold cathode mounting position should be set highly accurately.
A prior art method of highly accurately mounting cold cathode or like silicon chips or like very small elements will now be described. As shown in FIG. 8, Japanese Laid-Open Patent Publication No. 4-221865 discloses a technique of securing silicon chips or like very small element. This technique permits mounting and securing a plurality of very small elements automatically without need of positioning each element. Specifically, a coating material having satisfactory wetting property with respect to a solder material 84, is coated on the upper surface of a raised portion 82 of a base plate 81 and also on the lower surface of a very small element 83, and then the very small element 83 is set on the raised portion 82 via the solder material 84. By fusing the solder material 84 in this state by means of heating, the very small element 83 and the raised portion 82 pull each other due to the surface tension in the solder material 84. As a result, the very small element 83 is soldered to a predetermined position. In this technique, the final position of the very small element 83 is highly accurately determined absolutely by the shapes and sizes of the raised portion 82 and the very small element 83 and the surface distribution of the surface tension of the solder material 84 over the raised portion 82 and the very small element 83.
As shown in FIGS. 9(a) and 9(b), Japanese Laid-Open Patent Publication No. 1-108834 discloses a technique of preventing flux and gases contained in the solder from flowing out to portions where wires are secured to avoid deterioration of the mechanical strength, electric properties and thermal stability of these parts. Specifically, a metal base 91 has holes 92, in which leads 96 are secured by an insulating material 93. The leads 96 are connected to a printed wiring board 94 using a solder material 97. The flux and gases contained in the solder material have adverse effects on a chip 95 and bonding wires 98. Accordingly, a bank 99 is provided to prevent the flow-out of the flux to the chip 95 and bonding wires 98. As shown in FIGS. 10(a) and 10(b), it is further well known in the art to provide protuberances to engage with an element for the positioning thereof. Specifically, when securing a chip 102 to a base plate 101, two surfaces of the chip 102 are pushed against protuberances 102 to improve the accuracy of the mounting position.
The present invention seeks to solve the following problems in connection with the construction and method of manufacture of an electron gun using cold cathodes. (1) The cold cathode has to be mounted and secured in a predetermined position with an accuracy of plus and minus several .mu.m. (2) It is necessary, when sealing the electron gun in the cathode-ray tube with glass after the securing of the element, to prevent deterioration of the mounting position accuracy in a high temperature sealing atmosphere (of about 550.degree. C. at the peak) below the softening temperature of the glass.
The dimensional precision of the general silicon chip forming the cold cathod element will now be described as a principle subject. The silicon chip is obtained from a silicon wafer by dicing or like separating method. In the dicing process, the dimensional accuracy of the of outer shape of the chip is at most about several ten .mu.m, and the parallelness of the opposed surfaces is about 180.+-.5 degrees. These parameters are determined by the thickness reduction of the dicing blade used for the dicing as the blade is worn during the dicing of the wafer, as will as the accuracy of the opitical microscope or the like used initiual positioning for the dicing, mechanical accuracy of the dicing apparatus and so forth.
In the prior art cases of FIGS. 8 and 10, the dimensional accuracy of the outer shape of the chip is directly reflected on the chip mounting position accuracy. In the prior art case of FIG. 8, in addition to the dimensional accuracy of the chip it is sufficiently predicted that the wetting property of the solder has planar distributions on the side of the substrate and in the back surface. Therefore, it can be hardly thought that the raised portion and the chip center coincide, giving rise to the problem that the mounting position accuracy is deteriorated beyond the outer shape dimensions of the chip.
Gold-silicon eutectic alloys are typical autectic alloys used as the solder for bonding cold cathode elements on cathode structure substrates. The gold-silicon eutectic alloys are fused at about 430.degree. C. When sealing the electron gun in the picture tube, the temperature is raised to about 550.degree. C., which is higher than the melting points of the gold-silicon eutectic alloys. Therefore, even when the cold cathode element is highly accurately mounted at the time of the sealing of the tube with glass, its position accuracy is deteriorated due to its movement caused with the fusing of gold-silicon alloy solder.
A gold--gold thermal press process is a different general chip mounting process. Again in this case, the interface between gold and silicon is alloyed, so that the mounting position accuracy is deteriorated due to fusing of the mounting material at temperatures in the neighborhood of 550.degree. C., the temperature at the time of the sealing of the tube with glass.
Where eutectic alloys with melting points higher than 550.degree. C. are used as the mounting material, the following problems are posed. (1) Silver solder or like material results in great flux and other impurities and is therefore not suited for the mounting of semiconductor chips. (2) Where the melting temperatures are 600.degree. C. and above, the mounting position of the chip is deviated during cooling after the mounting of the chip due to the thermal expansion coefficient difference between the metal base and the chip mounted thereon, thus deteriorating the mounting accuracy. (3) Where the system is heated to a temperature of 600.degree. C. or above, the element may be damaged unless the oxygen concentration in the atmosphere is held low.